Major reserves of nickel are constituted by lateritic ores in which the nickel is in the form of oxide or silicate minerals. These ores are found in localized areas where suitable climatic and hydrological conditions have existed in past geological times to promote their "laterization" accompanied by a concentration of their nickel content.
The nickel content of these lateritic deposits varies over a wide range. While in the better deposits the average nickel content may reach and even exceed 2-3% nickel, it is in the range of 1-2% nickel in the great majority of the known lateritic nickel ore reserves. Many of these deposits have been processed commercially under suitable geographic and economic circumstances. In the majority of cases, however, only the higher grade ore bodies have been exploited, utilizing principally selective electric reduction smelting processes. In a few cases, hydrometallurgical processes, based on acid leaching or selective reduction followed by ammoniacal leaching, have been applied commercially to certain medium grade ore bodies.
Because of the fact that they must treat the unbeneficiated crude oil, the commercial processes presently used for the extraction of nickel from lateritic ores are affected by restrictive economic limitations concerning the minimum grade of ore required for profitable operation. Constant efforts have therefore been made over the years to develop beneficiation processes that would yield nickel concentrates of a suitable grade from these ores. The main purpose of thse efforts was to improve the profitability of the higher grade nickel lateritic ore bodies for which existing technology is available and to make it possible, besides, to treat economically lower nickel content laterites for which present extractive technology would not be commercially profitable.
The chloridation of base metal oxides in ores or calcines that are heated in contact with alkali and alkali-earth chlorides is known art and has been applied to a wide variety of metals.
Various processes have been developed in this respect for the recovery of the base metal chlorides formed. These processes fall into the two major categories:
A. Processes in which the base metal chlorides formed are caused to volatilize and are recovered selectively from the process gases for subsequent treatment and purification (e.g. the Kowa Seiko process applied to the purification of pyrite calcines containing base metals). PA1 B. Processes in which the base metals are precipitated selectively from their chlorides in the metallic form on carbonaceous reductants which are added to the ores or calcines simultaneously with the alkali or alkali-earth chlorides (segregation roasting). PA1 a. Iron oxides and coal or coke are mixed in suitable proportions, and after the addition of moisture and a binder like bentonite, if necessary, are agglomerated into miniballs of a diameter smaller than 6 millimeters and preferably finer than 10 mesh. PA1 b. The miniballs thus formed are processed in a suitable furnace (rotary kiln, multiple hearth furnace, or fluidbed roaster) in which they are heated up to a temperature of 700.degree.- 1000.degree. C at which they are maintained under suitable flame conditions for the time required to transform most of the iron oxide into metallic iron.
Processes falling in the second category have first been developed for copper oxide ores over the last fifty years under the name of "segregation processes".
Caron U.S. Pat. No. 1,487,145 teaches the extraction of the values of nickel and cobalt and copper from such ores by crushing the same and subjecting them to a reducing roast by a solid reducing agent with the addition of a small amount of sodium chloride. The typical operating temperature may be 750.degree.-800.degree. C, although higher temperatures are contemplated, and magnetic metallic nickel is produced.
Moulden and Taplin in British Pat. No. 250,991 describe a similar roasting operation in which a particular Chilean chrysocolla ore was admixed with a solid fuel and heated for one hour at a temperature of 700.degree. C in a vessel from which oxygen was excluded. It was found that the copper migrated from the ore particles and was found in a finely divided condition outside them. Investigation disclosed that the ore naturally contained a small proportion of sodium chloride which was essential to the success of the process. Subsequently, other oxidized copper ores which did not naturally contain any halide were roasted in a similar manner with an addition of a small amount of sodium chloride or other halide and segregation of the metallic copper was similarly successfully achieved. Carbonaceous reducing agents which may be employed include bituminous coal, graphite, coke and wood charcoal. In the case of these copper ores, it was found that a roasting temperature of from 500.degree. to 700.degree. C was satisfactory.
Carlos M. Diaz, in his thesis "Mechanism of the Segregation Process and Its Potential Application to Nickel Ores", Department of Mineral Engineering, Columbia University, 1958, describe a large number of experiments and concludes that they demonstrate that the segregation process is suitable for treating oxidized nickel ores.
The history of the segregation roasting process as applied particularly to copper ores is discussed in some detail by M. R. W. Rey in a paper entitled "Early Developments of the Copper Segregation Process" appearing in Volume 76 (1967) of the Transactions of the Institution of Mining and Metallurgy.
U.S. Patents which discuss the application of the segregation roasting process to nickel ores include U.S. Pats. No. 2,995,455, No. 3,033,671, No. 3,148,974 (dealing princpally with copper ores), No. 3,323,900 and No. 3,453,101. French Patent No. 1,101,906 deals with the segregation roasting of low nickel content ores including a calcined roast above 1000.degree. C to obtain large particles, but below 1200.degree. C to avoid sintering. Canadian Pat. No. 848,377 discloses a segregation procedure generally analogous to that used for the winning of copper from refractory oxide-silicate copper ores.
In the papers published by I. Iwasaki, Y. Takahashi, and H. Kahata entitled "Extraction of Nickel from Iron Laterites and Oxidized Nickel Ores by a Segregation Process", Transactions of AIME (1966) and by K. Nagano, K. Kojima and Y. Takahashi entitled "Extraction of Nickel from Low Grade Nickel Ores by Segregation Processs", Ninth International Mineral Processing Congress, Prague (1970), they report that they conducted a series of experiments with satisfactory results on the recovery of nickel, applying the concept of the segregation process as employed in the treatment of oxidized copper ore or copper silicate ore. Roasting temperatures between 750.degree. C and 1100.degree. C are mentioned, and the halide bearing material may be CaCl.sub.2 or NaCl. Metallic nickel precipitates on the solid reducing agent in the same manner that copper is described as being deposited on the carbonaceous reducing agent in British Pat. No. 250,991.
The segregation roasting of refractory copper ore has been commercially developed by use of the TORCO Process described by E. T. Pinkney and N. Plint in the Transactions of the Institution of Mining and Metallurgy, Vol. 76 (1967) "Treatment of Refractory Copper Ores by the Segregation Process", and Pinkney, "The TORCO Process -- recent developments", as well as by K. E. Mackay and N. Gibson, "Development of the pilot commercial TORCO plant at Rhokana Corporation, Ltd., Zambia", Vol. 77 (1968). However, the segregation roasting of nickeliferous lateritic ores does not appear to have achieved any widespread commercial success and it is believed that techniques should be employed which differ from those successfully utilized with the copper ores.